DESCRIPTION (Adapted from Applicant's Summary): The long term objectives of this research are to elucidate the mechanisms of viral membrane fusion proteins and to use this information in the rational design of novel anti-viral agents. There are two types of viral fusion proteins. One, typified by the influenza hemagglutinin (HA), functions at low pH for endosomal virus entry. The other, typified by the envelope (env) glycoprotein of HIV, functions at neutral pH for cell surface virus entry. When exposed to low pH, HA undergoes conformational changes, exposes its fusion peptide, binds to target membranes, induces hemifusion, and then opens a fusion pore. For retroviral env proteins (which function at neutral pH) interactions between env and host cell receptor(s) trigger fusion-inducing conformational changes. The first hypothesis of this proposal is that conformational intermediates between the native fusion protein and its final form mediate distinct steps of the fusion cascade. The second hypothesis is that after activation, all viral fusion proteins use a common mechanism. A third hypothesis is that small molecular weight anti-viral agents can be designed that block distinct steps in the fusion process. The major goal of this proposal is to test these hypotheses for a viral fusion protein that functions at low pH, the influenza HA, and for one that functions at neutral pH, the env glycoprotein of a model avian retrovirus. The specific aims are to: 1) test a new model for HA-mediated membrane fusion, 2) elucidate the fusion mechanism of the avian retroviral env protein, and 3) use a structure-based approach to identify small molecules that prevent the fusion-inducing conformational change in the influenza HA.